A study on the fish assemblage of the Ipanema River, a small affluent of the Tietê River basin in southeastern Brazil, was performed aiming to look for structural patterns of species diversity in small lowland lotic environments. Fish samplings were performed every two months from June 2003 to April 2004 at four sample sites located on the lower stretch of the river. Local assemblage showed to be species rich, with fifty-two species belonging to Characiformes (25 spp.), Siluriformes (19 spp.), Cyprinodontiformes (3 spp.), Gymnotiformes (2 spp.), Perciformes (2 spp.), and Synbranchiformes (1 sp.). Fish fauna was composed of small-sized species (<200 mm SL) and by individuals of medium (up to 400 mm SL) to large (more than 400 mm SL) sized species. The Ipanema River, such as other small lotic transitional environments in the upper Paraná River drainage, is considered important for conservation of fish fauna because they cover available habitats for persistent populations of small-sized species and for non-persistent individuals or shoals of medium and large-sized fish species, which occupy other habitats along their life-history (e. g. floodplains, oxbow lakes, main channel of great rivers). The importance of the Ipanema River basin for fish fauna conservation is also reinforced by the fact that it is located in a highly impacted region of southeastern Brazil, near the São Paulo metropolitan area.
community structure; body size; conservation; neotropical; streams; upper Paraná River